The crystallisation stage of sugar recovery and manufacture from clarified, concentrated cane or beet juice is nowadays commonly performed in continuous pans. Most of these continuous pans are designs of the “horizontal type”. These incorporate a horizontally arranged calandria, into which steam or vapour is fed to cause heating and boiling of the massecuite (sugar crystals and molasses). As the massecuite boils in the calandria, vapour (steam) bubbles form. These bubbles and the higher temperature lower the massecuite density, inducing the massecuite to circulate up through the calandria. The vapour separates at the surface and the massecuite then returns down around the calandria through an open, unheated “downtake” passage, into the zone beneath the calandria.
Vigorous circulation is highly desirable both for even crystal growth and for good heat transfer from the calandria.
In most conventional pan designs, the zone beneath the calandria is bounded by the cool outer shell of the pan vessel. This cools the adjacent massecuite, causing two problems:                Density of the massecuite is increased in an area where upflow is desired, and        A layer of sugar /massecuite may settle and solidify in this area.        
The encrustation on unheated surfaces is of particular concern and has been reported in technical literature on continuous pans.
In one well-known pan as described in EP-A-0065775, the invention comprises two or more similar cells of a vacuum pan stacked one upon another. Each cell is supplied by active heating steam from a common supply i.e. at a common pressure and temperature. Each cell boils with the spaces above the massecuite interconnected, and therefor the spaces have passive steam at a common pressure. In the lowest cell, the space below the massecuite is surrounded by the external atmosphere. In the cells other than the lowest one, the space below is surrounded by the passive steam of the cell below. The passive steam is at a temperature lower than that of the masseuite above and cannot therefor promote circulation or induce boiling as required in the present invention.
The pan described in EP-A-0065775 requires the use of mechanical stirrers to promote circulation and avoid encrustation and build-up on these lower, cooler surfaces. This requirement is obviated by the arrangement of the present invention.
It is an object of this invention to provide an arrangement which reduces or at least prevents increased massecuite density and solidification due to cooling in the zone beneath the calandria.